The present invention relates to an apparatus for detecting a press-bonded ball at bonding portion in a bonding apparatus and a method for detecting a press-bonded ball at a bonding portion.
In a wire bonding method, a ball is formed at a tip end of a wire passing through a capillary, and the ball is bonded onto a first bonding point of a workpiece to form a press-bonded ball. Then, the wire is fed to bond the wire onto a second bonding point.
A wire bonding apparatus that performs such wire bonding includes a heat block capable of heating a lead frame, and a bonding arm that holds the capillary is positioned above the heat block. Further, the wire bonding apparatus is provided with various heat sources such as, in addition to the heat block, an ultrasonic wave oscillator, an X-axis motor and a Y-axis motor that drive an X-Y table, a Z-axis motor that moves the bonding arm either in a swinging manner or upward/downward.
Variations in ambient temperature and operating heat generation caused by these heat sources produce a difference between thermal expansion of the bonding arm and thermal expansion of a detection camera holding arm that holds a detection camera, and an offset (in both amount and direction) between a central axis of the detection camera and a central axis of the capillary changes. The difference due to the change then appears as displacement of a bonding position. Detection of the displacement of the bonding position is typically performed, using the detection camera, by detecting a central position of a press-bonded ball that has been bonded on a pad.
A conventional method for detecting a press-bonded ball that has been bonded to a pad is disclosed in Japanese Patent Application Unexamined Publication Disclosure No. 8-31863 (Japanese Patent No. 3235008), for example. This method includes detecting the edges of a press-bonded ball based on at least three directions that respectively connect the indication point that has previously been indicated and that is for indicating a wire-bonding portion when wire-bonding (center of the pad) and points on an outer circumference of the press-bonded ball, and calculating the central position of the press-bonded ball based on the at least three edges of the press-bonded ball. Then, positional displacement of the press-bonded ball is calculated based on the difference between the indication point and the central position of the press-bonded ball. FIG. 6 shows a specific example of a wire bonding apparatus 100 with a lead frame 101, a semiconductor chip 102, and a workpiece 103. In the wire-bonding method described above, generally, amounts of displacement of at least two fixed points on the semiconductor chip 102 and at least two fixed points on the lead frame 101 from their respective normal positions are detected using a camera 111. Then, based on the values of the detected amount, bonding coordinates that have been previously stored are corrected using a calculation unit. When the detection is carried out using the camera 111, an X-axis motor 112 and a Y-axis motor 113 are driven so that the central axis 111a of the camera 111 comes immediately above the detection point. After the coordinates are corrected in the above described manner, a capillary 115 is moved in the directions along the XY axis and the Z axis, and a wire passing through the capillary 115 is wire-bonded onto a pad of the semiconductor chip 102 and a lead of the lead frame 101.
In this case, the central axis 111a of the camera 111 is offset by a distance W from the central axis 115a of the capillary 115. Accordingly, after detecting the displacement of the fixed points using the camera 111 and correcting the bonding coordinates, the XY table 116 is moved by an offset amount W by the X-axis motor 112 and the Y-axis motor 113, and the capillary 115 is positioned above the first bonding point of the workpiece 103. Subsequently, the wire is wire-bonded at the corrected bonding coordinates, based on the movement of the XY table 116 in the direction along the XY axis by the X-axis motor 112 and the Y-axis motor 113, and the movement of the capillary 115 in the direction along the Z axis due to a capillary arm 117, which includes an ultrasonic horn and a bonding arm, moving either upward/downward or in a swinging manner by a Z-axis motor 114. The capillary arm 117 is swingably provided on the bonding head, and the camera 111 is fixed to the bonding head by a camera holding arm. It should be noted that Xw represents an X axis component in the offset amount W, and Yw represents a Y axis component in the offset amount W. As described above, the central axis 111a of the camera 111 and the central axis 115a of the capillary 115 are positioned with the offset amount W therebetween that has been mechanically determined. Therefore, it is possible to carry out the bonding accurately at the bonding position by, as described above, detecting the amount of displacement of the workpiece 103 by the camera 111 and correcting the bonding coordinates, and then sequentially moving the capillary 115, according to a predetermined program, by the offset amount W to the corrected bonding coordinates.
However, the wire bonding apparatus 100 includes a heat block capable of heating the lead frame 101, and the capillary arm 117 that holds the capillary 115 is positioned above the heat block. Further, the wire bonding apparatus 100 is further provided with various heat sources such as, in addition to the heat block, the X-axis motor 112 and the Y-axis motor 113 that drive the XY table 116 in the direction along the XY axis, an ultrasonic wave oscillator that is installed within the capillary arm 117, and the Z-axis motor 114 that moves the capillary arm 117 either in a swinging manner or upward/downward.
Variations in ambient temperature and operating heat generation caused by these heat sources produce a difference between the thermal expansion of the capillary arm 117 and the thermal expansion of the camera holding arm that holds the camera 111, and an offset (in both amount and direction) between the central axis 111a of the camera 111 and the central axis 115a of the capillary 115 changes. The difference due to the change appears as a displacement of the bonding position. In this method, the positional displacement of the press-bonded ball is considered as a variation in the amount of offset between the capillary and the detection camera, and the amount of offset is calibrated. Japanese Patent Application Unexamined Publication Disclosure No. 7-297220 (Japanese Patent No. 3101854) discloses an apparatus for detecting positional displacement of a press-bonded ball, which is an apparatus related to Japanese Patent Application Unexamined Publication Disclosure No. 8-31863.
However, the wire remains connected to the press-bonded ball. As a result, the edges of the outer circumference of the press-bonded ball are positioned behind the wire when the bonding portion is imaged from above using the detection camera; and this makes it difficult to determine the exact center of the press-bonded ball. Moreover, in recent years, the diameter of a press-bonded ball is closer to the diameter of a used wire, which increases the hidden area of the edge portion of the outer circumference of the press-bonded ball. As a result, the determination of the exact center of the press-bonded ball has increasingly become difficult.